Abstract
Anemia is an extremely common clinical problem in critically ill patients. A sizable proportion are anemic at admission, and the majority of the remainder become anemic during their intensive care unit (ICU) stay, with the likelihood of becoming anemic increasing with each additional day in the ICU. The anemia most often has a multifactorial basis resulting from hemodilution; blood loss; impaired erythropoiesis; iron deficiency; and medications, including antibiotics, angiotensin-converting enzyme inhibitors, calcium channel blockers, beta blockers, and chemotherapeutic agents, among others. Laboratory evaluation involves obtaining a complete blood count to confirm the diagnosis of the anemia and to use the red blood cell indices, particularly the mean cell volume, to classify the anemia into microcytic, normocytic, or macrocytic types. This then suggests potential causes of the anemia and guides additional work-up. A reticulocyte count allows assessment of the response of the bone marrow. Iron studies are needed when iron-deficiency anemia is suspected, and methylmalonic acid and homocysteine levels help distinguish macrocytic anemia caused by vitamin B12 deficiency from that of folate. Lactate dehydrogenase levels, serum bilirubin levels, the Coombs test, and a peripheral smear are required to evaluate hemolytic anemias. Because of its frequency, anemia must be prevented in all patients admitted to the ICU by limiting routine screening blood tests, using smaller collection tubes, and using blood conservation devices especially in association with arterial lines. The historical approach of transfusing in response to a transfusion trigger—traditionally a hemoglobin level of <10 g/dL—has largely been abandoned. The adverse effects of red blood cell transfusion are well recognized and based on quality evidence; a restrictive transfusion strategy with transfusion for a hemoglobin level <7 g/dL is now the standard. A higher threshold of 8 g/dL is recommended for patients with active cardiac disease. Individualization is needed in the elderly and those with traumatic brain injury. Novel strategies, including the development of physiologically acceptable blood substitutes, continue to be explored.